Value documents will be understood here and in the following to mean sheet-shaped objects that represent for example a monetary value or an authorization and are therefore not to be producible at will by unauthorized persons. They therefore have features that are not easy to produce, in particular to copy, whose presence is an indication of authenticity, i.e. production by an authorized body. Important examples of such value documents, which can have in particular one or more layers of film and/or paper, are coupons, vouchers, checks and in particular bank notes.
In case of repeated and in particular not very careful use, such value documents can have properties or a condition that are changed compared to new, unused value documents. In particular, value documents such as bank notes can become limp. Limpness is understood here to mean a condition in which the bank note has considerably less stiffness or easier deformability and/or less elasticity than in the mint condition.
This change of condition of value documents can strongly reduce or even rule out their processibility by machine. Thus, bank notes whose limpness exceeds a certain measure are characterized as no longer fit for use, removed from circulation and replaced by new bank notes. The condition can therefore be determined as “fit for use” or as “to be destroyed”.
To permit a large number of value documents to be evaluated with regard to limpness, it is necessary to have a method for checking the condition of value documents with regard to limpness that is as reliable, gentle and simple as possible and can be carried out by machine.
At the time of filing, the condition with regard to limpness is frequently checked using a mechanical method wherein for example a bank note is deformed, in particular slightly bent, and the sound development occurring upon deformation is detected and evaluated. This method provides reliable results, but the bank notes are worn further by the deformation.